I have been trying to figure out the same thing with my car since I installed my clutch. I reused the flywheel and pressure plate...Only replaced disk. I thought it was a miss or a hesitation at first but I norrow it down to a vibration. I know what you mean about it being hard to describe..I only notice it when I take off in first or rev the car slowly in neutral. I'm almost certain it's the flywheel or pressure plate. I will be pulling mine and replacing it soon...I'll post what I find.
matt
matt